A simple instability condition based on the Ziegler's continuum principles as applied to large plastic flow, is extended for delineating the regions of unstable metal flow during hot deformation of powder metallurgy materials, This criterion has been applied to the existing flow stress data of powder metallurgy superalloy IN 100 and compared with the reported microstructural observations. The optimum hot working conditions for the PM superalloy IN 100 are suggested.